This invention is generally directed to a method for forging Ni-base superalloys so as to produce a substantially uniform, large grain size microstructure. Specifically, the method comprises isothermally forging fine-grained Ni-base superalloy preforms at slow strain rates in a range of temperatures that are above the .gamma.' solvus temperature of the superalloy of interest. In a preferred embodiment, the method also comprises additional annealing of the forged article in a range of temperatures which are also above the .gamma.' solvus temperature followed by controlled cooling to a temperature below the .gamma.' solvus.
Advanced Ni-base superalloys, such as those used for turbine disk applications, are currently isothermally forged at relatively slow strain rates and temperatures below their .gamma.' solvus temperatures. This method tends to minimize forging loads and die stresses, and avoids fracturing the items being formed during the forging operation. It also permits superplastic deformation of the alloy in order to minimize retained metallurgical strain at the conclusion of the forming operation. However this method also can have substantial limitations. In particular, it can produce an relatively fine-grain as-forged microstructure having an average grain size on the order of about 7 .mu.m. Alloys forged in this manner also have a tendency to exhibit critical grain growth as discussed further below.
For advanced applications, particularly high temperature applications, it is desirable to be able to produce articles from Ni-base superalloys that have a grain size within the range of about 50-150 .mu.m to promote damage tolerance, such as crack propagation resistance and high temperature creep resistance. Also, in advanced applications such as turbine disks, it may be desirable to have location specific properties, such as a finer grain size in the bore for enhanced low temperature strength and low cycle fatigue (LCF) resistance, coupled with a larger grain size in the rim for damage tolerance and high temperature creep resistance.
Larger grain sizes may be achieved using related art techniques. One method for increasing grain size, and improving the properties described above, is shown schematically in FIG. 1. This method includes isothermal forging 60 at a subsolvus temperature (T.sub.SB) and slow strain rates as described above, followed by supersolvus annealing 70 at a temperature (T.sub.SP) in the range of 0.degree.-100.degree. F. above the solvus temperature, followed by controlled cooling 75. However, most Ni-base superalloys tend to achieve a grain size in the range of only about 20-30 .mu.m when processed in this way. Also, unless carefully controlled so as to avoid retained strain in the alloy after forging, this method is subject to the problem of critical grain growth, wherein the retained strain in the forged article is sufficient to cause the random nucleation and growth (in regions containing the retained strain) of very large grains within the forged article, from for example 300-3000 .mu.m. Isothermal forging followed by supersolvus heat treatment has been shown to produce a large grain size, in the range of 100-300 .mu.m, in Ni-18Co-12Cr-4Mo-4Al-4Ti-2Nb-0.035Zr-0.03C-0.03B, an advanced Ni-base superalloy also known by the tradename KM4. However, this particular result is not reproducible in Ni-base superalloys generally, but limited to this particular alloy composition. Also, grain sizes in the range of about 150-300 .mu.m are generally considered to be less desirable because of the attendant reduction in the low temperature strength of the alloy that is associated with these larger grain sizes.
Therefore, new methods of forging are required to produce articles having a controlled range of grain sizes as described above.